What is a COVID-19 Antigen Test?

Published on: September 10, 2020

Written by: Dr. Po-Chang Hsu, MD

A COVID-19 Antigen Test is a medical device that identifies the presence of viral proteins in a biological sample, such as saliva or cell tissue swabbed from the nasal cavity. Antigen tests are less expensive than PCR (molecular) tests, provide results in minutes, and similarly to other genetic tests, reveal active infection. Antigen tests have been used for quite sometime now to detect strep throat, influenza, tuberculosis, HIV, and other contagious diseases.

There are many types of COVID-19 tests to determine whether a person is or has been infected with the SARS-CoV-2 virus. Currently, the three most popularly used testing devices are the reverse-polymerase chain reaction (RT-PCR) test, the antibody test, and the antigen rapid swab test. This article will focus on the antigen swab test.

How Does a COVID-19 Antigen Test Work?

The COVID-19 antigen test detects the SARS-CoV-2 virus using the concepts of both the antigens and antibodies. The antigen test uses antibodies that other animals produce to detect antigens found on coronavirus’s surface.

The sample collector first collects the mucus containing the virus by swabbing the back of a person’s nose or throat. The sample collector then dips the swab into a liquid solvent to dissolve the mucus to release the viral particles. The solvent with the viral particles is then applied to the test container that is coated with antibodies specific to the coronavirus. The antibodies then bind to the coronavirus antigens in the solvent.

The sample collector can then apply a second antibody mixture solution to the test container. The antibodies in the second mixture solution contain a dye that makes them visible to the eyes or fluorescent light. If the sample contains viral antigen proteins, those antigens will bind to two types of antibodies, one that attaches the antigen particles to the test container and another one that makes the antigens visible. The higher the number of antigen particles present in the sample, the more visible they would be, suggesting that the patient has a coronavirus infection.

On the other hand, if there is no or not enough viral proteins, there would be no antigens for the antibodies to bind to, and there would be no visible dye in the test container, suggesting that there is no coronavirus infection in the patient.

What are the Benefits and Drawbacks of Antigen Testing?

The benefits of antigen testing are numerous. The most obvious ones are its cost and speed. While an RT-PCR test may take hours to days to perform, most antigen tests can give results under an hour, sometimes as short as 15 to 20 minutes. The antigen tests are cheap to perform as they do not require trained technicians or expensive machines like the RT-PCR test. Also, while RT-PCR requires specialized machines and is limited by how many tests each machine can run in a set amount of time, the antigen tests are only limited by the number of kits available at a given time. Thus, the antigen tests can screen a large number of subjects at a single time point, something that would be hard to perform with the RT-PCR tests.

The other benefit of antigen testing is that it fills the gap of early coronavirus detection prior to antibody formation (which is required for antibody testing accuracy). While the antibody test can detect whether a person has developed antibodies against coronavirus, the antibodies usually develop 2 weeks after the infection. On the other hand, the antigen testing can detect the presence of coronavirus as soon as there is enough virus in the sample, which occurs during the early phase of the infection before the antibody formation two weeks after the infection.

There are drawbacks to the antigen testing as well. One of the drawbacks is that the antigen test is not as sensitive as the RT-PCR test. The antigen test depends upon the number of viruses that is present in the sample and may not give a positive result unless there are enough viral particles in the sample. The RT-PCR takes time to amplify the genetic material in the sample and has a lower threshold than the antigen test. The antigen test thus has a higher false-negative rate compared to the RT-PCR test, meaning that a patient who has an infection may have a negative result with the antigen test when the RT-PCR test can show a positive result.

In Summary

The antigen test can be effective for the early screening of COVID-19 infection in a patient and is faster and cheaper to perform than the RT-PCR test. The cost savings and reduced man power needed to administer antigen tests makes it more suitable for screening a large number of subjects. However, it should also be noted that the antigen test can have a higher false-negative rate compared to the RT-PCR test and thus should not be used as the definitive test for COVID-19 infection. To know how the antigen and the antibody tests complement each other, click [here] (Link to the article of how antigen and antibody tests work together).